On PHPMaster.com Timothy Boronczyk has written up a tutorial about using the Slim microframework as a sort of "middleware" in your application - a wrapper around other functionality with an easier to use interface.

Slim is a microframework that offers routing capabilities for easily creating small PHP applications. But an interesting, and powerful, feature is its concept of Middleware. [...] I've found middleware to be an eloquent solution for implementing various filter-like services in a Slim app, such as authentication and caching. In this article I'll explain how middleware works and share with you a simple cache example that highlights how you can implement your own custom middleware.

He talks about what "middleware" is (complete with illustration) and how Slim can be used as a layer in the middleware stack. His example is a caching layer, based on Slim, that takes a request, checks the cache for it and returns it if it exists. If not, it saves the content to a database. He also includes code examples of how to use the "add" method to introduce your middleware libraries into the Slim application.